7 Secret Ways America’s Stealth Armada Stays Off the Radar

It's no secret how America's stealth warplanes primarily evade enemy radars. Their airframes are specifically sculpted to scatter radar waves rather than bouncing them back to the enemy. Somewhat less important is the application, to select areas, of Radar Absorbing Material (RAM) meant to trap sensor energy not scattered by the plane's special shape.

In short, the four most important aspects of stealth are "shape, shape, shape and materials," to quote Lockheed Martin analyst Denys Overholser, whose pioneering work resulted in the F-117 Nighthawk, the world's first operational stealth warplane.

But in addition to shaping and RAM, the Pentagon's current stealth planes -- the B-2 Spirit bomber, the F-22 Raptor fighter, the RQ-170 Sentinel drone and the in-development F-35 Joint Strike Fighter -- boast other, lesser-known qualities that help them avoid detection. (We left the Army's stealth helicopter out of the discussion owing to a lack of information.)

These other stealth enhancements include: chemicals to eliminate telltale contrails; sophisticated, untraceable sensors and radios; specially designed, hard-to-detect engine inlets; radar-canceling paint; and cooling systems for reducing a plane's heat signature. All of these evasion methods have been disclosed by the Air Force, although sometimes in scant detail.

With China and Russia both demonstrating a rapidly improving grasp of stealth shaping -- and materials to a lesser extent -- these other, possibly harder-to-master aspects of radar-evasion are arguably becoming more important to maintaining America's aerial advantage.

Smart Sensors

Radar is like long-range eyes in the sky for modern warplanes. Without this sensor, a plane is more or less flying blind. The problem is, radar works by emitting energy -- lots of it. And that can be detected by an enemy's own passive radar receptors in the same way that someone standing in a dark room can track the movements of another person carrying a flashlight.

The F-22, F-35 and B-2 work around this problem by practicing what Aviaton Week stealth guru Bill Sweetman called "emission-control principles." With the Raptor, emissions from the jet's APG-77 radar "are managed in intensity, duration and space to maintain the pilot's situational awareness while minimizing the chance that its signals will be intercepted." In other words, the plane's software is smart enough to use just enough energy to find and track targets -- and no more. The B-2 and F-35 have electronically-scanned radars that are similar to the Raptor's and probably employ the same tactics.

Plus the Raptor and Joint Strike Fighter both have non-emitting backup sensors that can fill in the gaps in the radar coverage. The F-22's ALR-94 radar-warning receptors are among the most sensitive ever designed and can accurately, and "silently," detect most radar-using targets at long range. The F-35 boasts a powerful set of cameras that achieve the same effect.

Photo: Air Force

Radio Silence

A stealth plane's communications could also betray its location. In the case of the RQ-170, the dish for the drone's satellite radio hardware itself is a possible give-away, as its antenna is potentially highly "reflective," or non-stealthy. It could be that's why Lockheed Martin designed the Sentinel spy drone with two distinctive humps on its back, each apparently containing a separate satellite dish. "If your UAV is being illuminated by radar, you turn to place that radar on one side of the aircraft and use the antenna on the opposite, 'shadow' side of the aircraft to communicate," Sweetman explained.

In the case of the B-2, F-22 and F-35, the bigger problem is how to communicate with other planes without sending out some obvious signal that can be tracked back to the source. Voice radio is out of the question. "As soon as I talk, I give myself away," said Mike Therrien, an Air Force comms expert. Likewise, non-voice radio datalinks used by older jets are too easy to detect. Lockheed installed on the 187 Raptors a short-range, low-power datalink that is minimally detectable. And the Joint Strike Fighter is getting a new, purpose-made, stealthy datalink that's also being added to the B-2.

But both of these links have problems interfacing with older comms networks, sometimes requiring stealth warplanes to be accompanied by special EQ-4 drones or E-11 manned planes with radio translation systems installed.

Photo: Press TV

Stealth With an 'S'

One of the biggest radar giveaways is inside an aircraft. In most planes, the engine turbines are visible through the air inlet -- and they're a huge source of radar reflectivity. To mask the turbines, stealth warplane designers must connect the inlet to the engine indirectly, by snaking the inlet duct inside the fuselage in a rough S-shape.

The S-shaped inlet is a tricky bit of engineering to pull off. Boeing refined its engine-obscuring techniques using a futuristic, one-off test plane called the Bird of Prey, among whose most important features was a very stealthy inlet. But the Lockheed-built RQ-170 is apparently too short for a curved duct and instead relies on a radar-blocking grill that covers the inlet mouth. Otherwise, the serpentine inlet is practically standard on current U.S. and, apparently, Chinese jets, but surprisingly Russia's T-50 stealth prototype doesn't have them.

Photo: Boeing

Chilling Out

Airplanes generate a lot of heat. And even if you completely mask a plane's radar signature, it might still give off telltale infrared emissions, especially around the engine exhaust but also from electronics, moving parts and surface area exposed to high wind friction.

The B-2 and F-22's flat engine nozzles spread out the exhaust to avoid infrared hot spots, but to save money all 2,400 planned U.S. F-35s will have a traditional, rounded nozzle that spews a lot of concentrated heat. The Spirit, Raptor and Joint Strike Fighter apparently all feature gear for cooling hot leading edges such as the fronts of wings. They also boast systems that sink much of the heat generated by the on-board electronics and actuators into the fuel. The F-35 in particular pushed that concept to the extreme. "We’re out of heat-sink capacity for the F-35,” said James Engle, a former Air Force deputy assistant secretary.

Some researchers have considered new fuel types with better thermal properties in order to boost the heat capacity of today's planes. One university study (.pdf) found that standard JP-8 jet fuel derived from coal instead of petroleum could safely absorb more heat.

Photo: Lockheed Martin

Skin Deep

For U.S. stealth warplanes, a paint job is about more than good looks. Stealthy Spirits, Raptors, Joint Strike Fighters and presumably Sentinels are coated in special paints that suppress heat and partially cancel out radar waves. But to work correctly, the paint has to be maintained in immaculate condition. "We are working all day every day," said Staff Sgt. Matthew Duque, an Air Force F-22 paint technician. Increasingly, high-tech robots guided by laser sensors are taking over stealth-painting duties.

In any event, the paint on the F-35 is designed to be more robust than that on the Raptor. The Air Force has such high hopes for the new pigment that it is also painting some of its F-16s with the same formula, hoping to lend the older jets a degree of stealthiness.

Photo: Lockheed Martin

Contrail Control

Contrails are formed when jet engines spew sulfur, nitrogen, tiny fragments of metal and other impurities into the atmosphere, attracting vaporized water that adheres to the pollutants and forms long, linear clouds that are visible for many miles in all directions, sometimes even at night.

That's obviously a problem for infiltrating warplanes trying to remain invisible to enemy defenders. In 1994 the Air Force paid Northrop Grumman $16 million to add a "contrail management system" to the 20-strong fleet of high-flying B-2 stealth bombers. The system somehow chemically prevents water from sticking to the bombers' exhaust, erasing any contrail. "How do those work? Beats the Hell out of me," Matt Rasmussen wrote in an article on the phenomenon.

It's unclear if any of America's other stealth warplanes have similar contrail-suppression gear, but it wouldn't be surprising if they did.

Photo: Air Force

Stealth Surprise

Perhaps the most remarkable quality of America's stealth warplanes is their continuing ability to escape public notice during years or even decades of development, testing and initial operations. The F-117 and B-2 were secrets until the Air Force didn't want them to be anymore. The F-22 and F-35 have always been highly visible programs, although many of the jets' specific capabilities are classified. The RQ-170, by contrast, reportedly flew during the 2003 Iraq war without any outsiders realizing what it was, and stayed in the shadows until a lucky photographer finally spotted one of the 20 or so Sentinels in Afghanistan in 2007.

Today the Air Force is apparently designing or testing at least two new, radar-evading drones plus the new Long Range Strike Bomber, an even stealthier successor to the now-25-year-old Spirit. But the only evidence of these classified programs is oblique references in financial documents, vague comments by industry officials and the occasional revealing commercial satellite photograph. Who knows what new qualities the next generation of stealth planes might possess in addition to those of the current armada.